Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that improves power delivery performance and fuel economy by achieving at least nine forward speed stages and widening speed ratio (or gear ratio) with a minimum number of constituent elements being used, and secures linearity of step ratios.
Description of Related Art
The recent increase in oil prices has caused car makers to meet global demands for improving fuel efficiency.
Therefore, research is being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and research is also being conducted to ensure both drivability and competitiveness for maximizing fuel efficiency by implementing an automatic transmission with multiple speed stages.
However, in the case of the automatic transmission, the number of internal components is increased as the number of gear shift stages is increased, and as a result, the automatic transmission may be difficult to mount, the manufacturing cost and weight may be increased, and power transmission efficiency may be deteriorated.
Accordingly, development of a planetary gear train which may achieve maximum efficiency with a small number of components may be important in order to increase a fuel economy enhancement effect through the multiple-speeds.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, since a conventional 8-speed automatic transmission has gear ratio span of 6.5-7.5, improvement of power performance and fuel economy may not be great.
In addition, if an 8-speed automatic transmission has a gear ratio span larger than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speed stages is necessary.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.